Below I have broken down the steps involved in making a custom trim like this. The steps and process can be applied to any plastic-to-plastic fabrication and not just this air suspension controller mount.
Before you start cutting anything up just stop and study the shape of the fascia and mount, visualize the end result and come up with a plan. If you come up with a plan now and do the job in your head, you’re going to know if there are any awkward parts to the job like tabs that need to stay in place or thin edges that could go out of shape when you make a cut and in general the process will go a lot smoother.
Once you’ve studied the parts, you need to trim down the mount and make the opening in the fascia bigger so the mount nest’s inside the new opening. If you’re using one of my mounts, a tip for trimming it down is to score a cut line on the flat area with a blade and ruler. If you score it enough times you’ll be able to snap the excess off leaving a clean straight line. This is easier, quicker and less messy than trimming it down with a Dremel. At this point, I like to take a sanding block with some 120 grit paper to the edges to straighten them out if needed and generally fine-tune the shape that I want.
Once you’ve trimmed down the mount, you want to draw around it with a marker pen on the fascia so you’ve got a guideline to make the cut. You want to be aiming for around a 2-3mm gap between the mount and the fascia when it’s in position so that there’s enough room for glue.
Once everything’s marked out, double-check the opening you're about to cut is straight and the positioning looks right. For cutting the opening I use a Dremel 3000 with a cutting disc to remove the bulk of the material, then use a sanding drum to make any further adjustments. It’s always worth offering the mount into the opening as you're making these small adjustments to make sure you’re not taking off too much material.
Once I’m happy with the size of the opening I move onto keying the surfaces that I need to the glue to bond to. This being the front and back of the fascia and the front and back of the mount, basically any area that glue is going to make contact with. This is an important step that you want to be a thorough as you can with, as this will determine how strong of a bond you’ll have, and reduce chances of the joint cracking over time. I tend to stick to 3 main Dremel bits, the cutting wheel, drum sander and this cone-shaped one, although I do use this ball end bit for tight areas.
Once you’ve keyed all the surfaces, I wipe down all the areas with a fresh microfiber cloth and IPA, making sure any loose bits of plastic have been removed as well.
Now that everything is clean and ready to go, offer the mount into the opening, line it up exactly where you want it and pin it into position using CA glue. This is basically a super glue with an activator spray, we’re only using this to get our position right before the full glue-up. I like to pin the 4 corners first, then once I’m happy with the positioning I’ll go around all of the edges and pin it a couple more times.
At this point, we need to be thinking about the shape of the final piece and how we want to two surfaces to transition into one another. We need to look at any high spots and taper those edges to the same level as the lowest points, so when we come to the filler stage, those high spots won’t come through when we’re sanding. I like to use the cone shape bit for this as the shape lends itself to what we’re trying to achieve.
Once the surfaces run into each other at the same level, we want to carry on with the Dremel and V the edges where they meet. This is also an important step as this shape is what will give us a strong joint. By V’ing and tapering the edges, we’ve created an area where the glue will lap over the two surfaces. As the glue will be over the back two surfaces, between the surfaces and on top of the surfaces, the glue will completely lock these surfaces together as the two parts aren’t able to shift forward or back, which is how cracks form over time.
Once all that prep is done, I like to go around with some 120 grit paper for good measure and knock off any loose plastic. Another thing to watch out for is where the surfaces meet, the plastic will be thin now the joint has been V’d. If I can see any really thin bits that are loose but still connected, I like to take a blade and trim those off, as well as opening up any joining areas that have closed up with melted plastic using the cutting disc.
Again, the next step is to clean the surfaces with my IPA and cloth making sure it’s as free from loose plastic and dust. Now we’re ready for glueing. I use a two-part plastic repair adhesive, there are various options available, I have used a variety of different ones myself like the 3m 50901, but any automotive bumper repair glue will work fine. Make sure if it’s a new tube, to watch the glue coming through the nozzle to ensure both parts are coming out equally, these two-part adhesives do have a tendency to block and only one side will come out, so I like to apply some to a scrap piece of paper and make sure it’s mixing properly before applying it to the part, the last thing you want is for there to be areas of uncured glue.
Once the glue has dried I will use the cone bit again to smooth out the glue to make it flat and also making sure the glue is lower than the two surfaces so it doesn’t come back through when we’re sanding through our filler. This is also a reason why we V’d the joint, because if we left the joint without that taper, then removed the high spots of the glue, we would be left without glue lapping over onto the top of the two surfaces, meaning that if the joint did shift it would very likely crack.
Once it’s all shaped, sanded down with some more 120 grit, cleaned and prepped, we can move onto fillers. I only use two fillers. Upol Plast x 6 and Upol dolphin glaze. The Plastx is what I use to build the bulk of the shape, then the Dolphin glaze to refine the shape and fill any pinholes. I use plastx over regular body fillers or fibreglass because it’s specifically designed for plastics. In a vehicle, there is a constant cycle in temperature changes from hot to cold which can be quite drastic from summer to winter and even day to night. As plastics expand with heat and contract with cold, a regular body filler or fibreglass is more likely to crack during these cycles as they’re rigid, the Plastx, on the other hand, is a flexible filler, so it’s able to shift with these movements reducing the chances of any cracking. This is probably me being overly cautious as these shifts are going to be minimal, but to me, it makes sense and a step worth taking in my opinion.
Try to be conservative with how much filler you apply, if you need to build a lot of height for shaping, do this in stages rather than a large amount in one go, this will reduce the chances of getting any air traps in the filler. Also, it’s much easier to add more filler than having to sand huge lumps off. Always mix on an onion pad or a non-absorbent surface, if you mix your fillers on a scrap piece of wood or cardboard, the resin in the fillers will be absorbed compromising the properties of the filler.
The next stages are just back and forth with filler and sanding, working up the paper grits until you’ve got the shape that you want. My process typically is filler, 120 grit, filler, 220 grit, filler, 400 grit, filler, 400 grit. This is assuming that the shape and levels of the filler are right before moving up and changing grits. We move up the grades of paper to take out the sanding scratches made from the previous grit, eventually giving us a smooth shaped surface. Make sure that before applying any filler, the surface has been thoroughly cleaned with IPA, if it’s not then you’ll be laying filler over filler dust so the filler won’t adhere to the surface and cause you problems further down the line.
What you want to be seeing is when going through this process is the filler feathering out into the fascia’s surface and the mounts surface, if you're seeing this then you will have a nice smooth transition between the surfaces, if you're seeing any hard edges of either the fascia or the mount, I like to take the Dremel with the cone bit and taper that down and re-apply filler to that area. These hard edges may look flat to the filler’s surface, but once you’ve applied paint it will almost definitely show through.
Once you’re happy with the shape, you can either lay down a guide coat, which is a coloured coating you can apply to the surface which reveals any low spots in the filler when you take sandpaper to it, or it will reveal a high spot, this is for you to assess as each situation is different. The other option is to lay down a light layer of primer like I do and use that in the same way, if after doing this there are any low or high spot, sort those out before moving onto laying down a proper coat of primer.
Sometimes after the primer is on the surface, the overall shape is much easier to see with it being one consistent colour, so it’s much easier to spot any imperfections, pinholes or areas that need further shaping.
Once you’re absolutely happy with the shape, clean the surface off and lay down that coat of primer and then paint.
That’s pretty much it, once it’s dry get it fitted into the vehicle, mount the controller and if you’re like me you’ll sit and just stare at it for a while and appreciate the fact you've created a one-off piece by hand.
Thanks for taking the time to read this. Hopefully, this paired with the video has given you some insight into the process of creating a custom part and encourages you to give this type of work a try! Patience is the key, take your time and ultimately have fun. It's a very rewarding process to create something by hand that you can look at and say "I made that".